There are many ways to line your pond and make it waterproof. The most common are flexible, rigid and concrete. Rigid liners are the easiest and least expensive to install but also most vulnerable to cracking. EPDM and PVC are the most common types of liner.

PVC liner is the least expensive
and also the most difficult of the
two to work with since it is stiff
when cold, you need to lay it out in
​ the sun before laying it in your hole. PVC also gets brittle when exposed to direct sunlight for extended periods of time and may crack allowing water to escape. PVC cost about $0.60 per square foot and comes in black and gray and is about 20 mil thick. 

EPDM liner is thicker (about 45 mil)
and easier to form to your hole since
it does not need to be placed in the
sun to warm it up before laying. It is also
resistant to cracking when exposed to sun.
​EPDM costs around $1.06 per square foot. It comes in black and is usually sold by the running foot so pay attention to the width. If you need to splice or repair seams M1 sealant is a good choice. This link to EPDM supplies on the internet is a convenient source of supplies.

Rigid or fiberglass shells make the job
easy because they make your
decisions for you when it comes
to shape and depth. Keep in mind
if you are going to put koi in a pond
​most rigid ponds are not deep enough for them to survive. It is marginal for goldfish as well. These ponds cost $50 - $200 depending on size and shape. You must back fill and support the sides and bottom or cracking will occur during the winter when the water in the pond freezes. 

Concrete lined ponds are the most expensive and complicated ponds to make. You must use rebar and chicken wire as part of the construction as the changes from extreme cold to extreme heat will cause the concrete to crack from expansion and contraction allowing water to seep out. The lime in the concrete will also kill fish if not treated or sealed correctly. For these reasons it suggested that you do not install a concrete pond unless you are experienced in concrete work and understand the chemical processes involved.